The descending serotonergic neurons of nucleus raphe magnus have been proposed to play a crucial role in the expression of opiate and stimulation-produced analgesia. However, it is not clear what neurotransmitters might control the activity of this analgesia-producing system. The present proposal will examine that question by a combination of anatomical and electrophysiological means. The neurotransmitters adjacent to descending serotonergic neurons will be examined using a combination of retrograde labeling and two-color immunofluorescence. Additionally, the effects of these neurotransmitters on descending neurons will be examined using iontophoresis. These experiments will clarify how the activity of the serotonergic neurons within nucleus raphe magnus might be modulated.